Already known is a class of electron beam devices providing electron beams of variable energy and velocity. A change in the beam energy brings about a change in the colour of the luminescence, the duration of the afterglow, or other characteristics of the screen. The operating principle of such devices is based on the relationship between the depth of penetration of electrons of an electron beam into a solid and the energy of these electrons.
The screens of such electron beam devices may be of different types. There may be single-layer screens including particles of a material coated with other materials which serve as an energy barrier for electrons. The screen may be composed of several layers of different materials; some of these layers exhibit different properties when excited by electrons, whereas other layers serve as an energy barrier for electrons. When acting upon screens of the latter type, electrons of different energy levels penetrate into and excite different layers of the screen.
The operating principle of such screens makes it clear that the excitation of materials possessing different properties necessitates a certain increase in the energy of the electron beam's electrons.
However, the electron beam is defocused on the screen unless certain measures are taken.
Different techniques are already known which make it possible to produce a focused image on a screen with different electron beam energies (cf. UK Patent Specification No. 947,916, Cl. H4 D). In this patent specification, disclosure is made of a multi-gun electron beam shaping system, wherein each gun produces a focused electron beam possessing a certain amount of energy.
The above system is disadvantageous in that it contains a plurality of guns, which accounts for the complicated design of the system; in addition, images produced by different guns do not match on the screen.
There is also known an electron beam device with variable electron beam energy (cf. USSR Inventor's Certificate No. 441,612, U.S. Pat. No. 4,044,282), comprising a focusing system for constant focusing of an electron beam on a screen. The focusing system includes two axially symmetrical electrodes successively arranged across the path of the electron beam. The first of these electrodes is electrically coupled to an accelerating electrode intended for electron beam shaping; the second electrode is electrically coupled to the screen.
The focusing system of that electron beam device comprises a magnetic lens with a constant magnetic field intensity, and an electrostatic lens arranged with respect to the magnetic lens so as to compensate for changes in the electron beam focusing. The electrostatic lens is composed of two axially symmetrical electrodes successively arranged across the path of the electron beam. The first of these electrodes is electrically coupled to the accelerating electrode, whereas the second is electrically coupled to the screen.
The above electron beam device under review has all the disadvantages inherent in electromagnetic focusing systems, which include the complexity of aligning the magnetic lens in relation to the axis of the device, the large size and great weight of the device, the necessity of utilizing a sufficiently powerful source to energize the magnetic coil, etc. In addition, changes in the energy levels of the electrons of the electron beam make it hard to focus the electron beam; that means that for each electron beam device, one must find an optimum position of the middle plane of the magnetic lens with respect to that of the electrostatic lens, and appropriately select the potential of the accelerating electrode and the magnetomotive force of the magnetic coil.